/*
 * Copyright (C) 2007 Google Inc.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.common.collect;

import java.util.Collection;
import java.util.Set;

import javax.annotation.Nullable;

import com.google.common.annotations.GwtCompatible;

/**
 * An empty immutable set.
 * 
 * @author Kevin Bourrillion
 */
@GwtCompatible(serializable = true)
final class EmptyImmutableSet extends ImmutableSet<Object> {
    static final EmptyImmutableSet INSTANCE = new EmptyImmutableSet();

    private EmptyImmutableSet() {
    }

    public int size() {
        return 0;
    }

    @Override
    public boolean isEmpty() {
        return true;
    }

    @Override
    public boolean contains(Object target) {
        return false;
    }

    @Override
    public UnmodifiableIterator<Object> iterator() {
        return Iterators.emptyIterator();
    }

    private static final Object[] EMPTY_ARRAY = new Object[0];

    @Override
    public Object[] toArray() {
        return EMPTY_ARRAY;
    }

    @Override
    public <T> T[] toArray(T[] a) {
        if (a.length > 0) {
            a[0] = null;
        }
        return a;
    }

    @Override
    public boolean containsAll(Collection<?> targets) {
        return targets.isEmpty();
    }

    @Override
    public boolean equals(@Nullable Object object) {
        if (object instanceof Set) {
            Set<?> that = (Set<?>) object;
            return that.isEmpty();
        }
        return false;
    }

    @Override
    public final int hashCode() {
        return 0;
    }

    @Override
    boolean isHashCodeFast() {
        return true;
    }

    @Override
    public String toString() {
        return "[]";
    }

    Object readResolve() {
        return INSTANCE; // preserve singleton property
    }

    private static final long serialVersionUID = 0;
}
